As a method of irradiating an object with radiation and detecting the intensity distribution of the radiation transmitted through the object to obtain the radiographic image of the object, a technique has recently been developed, in which a digital image is acquired by using a photoelectric conversion apparatus in which pixels comprised of small photoelectric conversion devices and switching devices are arrayed in a matrix. Such radiographic apparatuses can immediately display acquired image data.
In a conventional radiographic apparatus, when the amount of radiation that reaches the apparatus enters a low region, the effect of quantization noise due to the decrease in arrival information amount and system noise inherent in the apparatus become large, and the S/N ratio of the image decreases. To ensure the minimum necessary quality of an acquired image, an X-ray automatic exposure control (AEC) circuit called a phototimer is used in order to obtain a minimum arrival radiation amount. As shown in FIGS. 8A and 8B, radiation detection devices used in the AEC circuit constitute two or three AEC radiation detection regions 5 so that the apparatus can be used for radiography of both chest and belly parts.
In a flat panel detector (FPD) which uses solid-state photodetection devices as image sensing devices of a radiographic apparatus, the AEC circuit is operated while arranging radiation detection devices separate from the FPD in front of it, as disclosed in U.S. Pat. No. 5,585,638.